Camshaft phasers (“cam phasers”) for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine's crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of an engine's camshaft.
U.S. Pat. No. 7,421,990 B2 discloses an eVCP comprising first and second harmonic gear drive units facing each other along a common axis of the camshaft and the phaser and connected by a common flexible spline (flexspline). The first, or input, harmonic drive unit is driven by an engine sprocket, and the second, or output, harmonic drive unit is connected to an engine camshaft.
A first drawback of this arrangement is that the overall phaser package is undesirably bulky in an axial direction and thus consumptive of precious space in an engine's allotted envelope in a vehicle.
A second drawback is that two complete wave generator units are required, resulting in complexity of design and cost of fabrication.
A third drawback is that the phaser has no means to move the driven unit and attached camshaft to a phase position with respect to the crankshaft that would allow the engine to start and/or run in case of drive motor power malfunction. eVCPs have been put into production by two Japanese car manufacturers; interestingly, these devices have been limited to very low phase shift authority despite the trend in hydraulic variable cam phasers (hVCP) to have greater shift authority. Unlike hVCP, the prior art eVCP has no default seeking or locking mechanism. Thus, phase authority in production eVCPs to date has been undesirably limited to a low phase angle to avoid a stall or no-restart condition if the rotational position of the eVCP is far from an engine-operable position when it experiences electric motor or controller malfunction.
U.S. patent application Ser. No. 12/536,575 and US Patent Application Publication No. 2011/0030632-A1 which are commonly owned by Applicant disclose an eVCP camshaft phaser comprising a flat harmonic drive unit having a circular spline and a dynamic spline linked by a common flexspline within the circular and dynamic splines, and a single wave generator disposed within the flexspline. The circular spline is connectable to either of an engine camshaft or an engine crankshaft driven sprocket circumferentially surrounding and rotationally fixed to the outside diameter of a housing, the dynamic spline being connectable to the other thereof. The wave generator is driven selectively by an electric motor to cause the dynamic spline to rotate past the circular spline, thereby changing the phase relationship between the crankshaft and the camshaft. The electric motor may be equipped with an electromagnetic brake. At least one coaxial coil spring is connected to the sprocket and to the phaser hub and is positioned and tensioned to bias the phaser and camshaft to a default position wherein the engine can run or be restarted should control of the electric motor be lost resulting in the electric motor being unintentionally de-energized or held in an unintended energized position.
Engine applications exist that require that the drive sprocket be made smaller in diameter than the housing to which it will be transmitting torque. The logical solution used in hydraulic type phasers is to place the small diameter gear axially behind the phasing mechanism, integral with a plate or rear cover, which is then fixed to the rear of the housing. However, implementing this solution with the eVCP presents two challenges. The first challenge is that offsetting the drive sprocket so far rearward would offset the radial drive load so far from the journal bearing that binding or wear problems could result. Increasing the length of the journal bearing to compensate for the offsetting of the drive sprocket is not practical due to the need for axial compactness. The second challenge is that the spring plate now needs to transmit the drive torque to the housing. The knurled press fit design presently known is not adequate to transmit this drive torque load.
While the eVCP does not rely on engine oil to actuate, it does rely on engine oil to lubricate the harmonic drive unit and bearings. In order to minimize parasitic oil pressure loss, the amount of oil flow used to lubricate the eVCP needs to be held to a minimum. This results in a dead headed oiling system in which there is not enough oil flowing through the eVCP to flush out contaminants. This allows the contaminants to accumulate within the eVCP which may lead to premature wear.
What is needed is an eVCP with a drive sprocket smaller in diameter than the housing which does not result in binding or wearing problems to the journal bearing. What is also needed is an eVCP that has a spring plate which is able to transmit the drive torque to the housing. What is also needed is an eVCP that prevents the accumulation of contaminants that may lead to premature wear of eVCP components.